I will assume you are wanting to message a group in a class and not a stand alone group created on the root or sub-account level. For the course group, it looks like a guide exists to address this issue. However, either I am terrible at following instructions or Canvas made some updates which changed the process. Either way, here is the guiding document How do I send a message to a user in a group in Conversations as an instructor? but I will also jot down the process that worked for me, which is different than the guide:
Open Canvas Inbox
Compose a new message
Select your course
Click on Address Book icon
Select Student Groups filter
Choose Group and select ALL or choose individual or multiple group members
You can learn how at https://community.canvaslms.com/docs/DOC-10506-4212716758
This process is the same regardless of user role.
I hope this helps,
Works, but it would be easier if Canvas would provide a list of students with tick-boxes (I wanted to send a file to 36 of 42 students and their should be an easier way) .....